Semiconductor light-emitting diode (LED) devices, which are primarily inorganic, have been made since the early 1960's and currently are manufactured for usage in a wide range of consumer and commercial applications. The layers comprising the LEDs are based on crystalline semiconductor materials. These crystalline-based inorganic LEDs have the advantages of high brightness, long lifetimes, and good environmental stability. The crystalline semiconductor layers that provide these advantages also have a number of disadvantages. The dominant disadvantages have high manufacturing costs; difficulty in combining multi-color output from the same chip; inefficiency of light output; and the need for high-cost rigid substrates.
In the mid 1980's, organic light-emitting diodes (OLEDs) were invented (Tang et al, Applied Physics Letter 51, 913 (1987)) based on the usage of small molecular weight molecules. In the early 1990's, polymeric LEDs were invented (Burroughs et al., Nature 347, 539 (1990)). In the ensuing 15 years organic-based LED displays have been brought out into the marketplace and there has been great improvements in device lifetime, efficiency, and brightness. For example, devices containing phosphorescent emitters have external quantum efficiencies as high as 19%; whereas, device lifetimes are routinely reported at many tens of thousands of hours. However, in comparison to crystalline-based inorganic LEDs, OLEDs suffer reduced brightness, shorter lifetimes, and require expensive encapsulation for device operation.
To improve the performance of OLEDs, in the late 1990's OLED devices containing mixed emitters of organics and quantum dots were introduced (Mattoussi et al., Journal of Applied Physics 83, 7965 (1998)). Quantum dots are light-emitting nano-sized semiconductor crystals. Adding quantum dots to the emitter layers could enhance the color gamut of the device; red, green, and blue emission could be obtained by simply varying the quantum dot particle size; and the manufacturing cost could be reduced. Because of problems such as aggregation of the quantum dots in the emitter layer, the efficiency of these devices was rather low in comparison with typical OLED devices. The efficiency was even poorer when a neat film of quantum dots was used as the emitter layer (Hikmet et al., Journal of Applied Physics 93, 3509 (2003)). The poor efficiency was attributed to the insulating nature of the quantum dot layer. Later the efficiency was boosted (to ˜1.5 cd/A) upon depositing a mono-layer film of quantum dots between organic hole and electron transport layers (Coe et al., Nature 420, 800 (2002)). It was stated that luminescence from the quantum dots occurred mainly as a result of Forster energy transfer from excitons on the inorganic molecules (electron-hole recombination occurs on the organic molecules). Regardless of any future improvements in efficiency, these hybrid devices still suffer from all of the drawbacks associated with pure OLED devices.
Recently, a mainly all-inorganic LED was constructed (Mueller et al., Nano Letters 5, 1039 (2005)) by sandwiching a monolayer thick core/shell CdSe/ZnS quantum dot layer between vacuum deposited inorganic n- and p-GaN layers. The resulting device had a poor external quantum efficiency of 0.001 to 0.01%. Part of that problem could be associated with the organic ligands of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) that were reported to be present post growth. These organic ligands are insulators and would result in poor electron and hole injection onto the quantum dots. In addition, the remainder of the structure is costly to manufacture, due to the usage of electron and hole semiconducting layers grown by high-vacuum techniques, and the usage of sapphire substrates.
As described in co-pending, commonly assigned U.S. Ser. No. 11/226,622 by Kahen, which is hereby incorporated by reference in its entirety, additional semiconductor nanoparticles may be provided with the quantum dots in a layer to enhance the conductivity of the light-emitting layer.
Quantum dot light-emitting diode structures may be employed to form flat-panel displays and area illumination lamps. Likewise, colored-light or white-light lighting applications are of interest. Different materials may be employed to emit different colors and the materials may be patterned over a surface to form full-color pixels. In various embodiments, the quantum dot LEDs may be electronically or photonically stimulated and may be mixed or blended with a light-emitting organic host material for hybrid inorganic-organic LEDs.
Both inorganic and hybrid inorganic-organic light-emitting diodes (LEDs) are electroluminescent technologies that rely upon thin-film layers of materials coated upon a substrate. These technologies typically and employ a cover affixed to the substrate around the periphery of the LED device to protect the device from physical harm. The thin-film layers of materials can include, for example, organic materials, quantum dots, fused inorganic nano-particles, electrodes, conductors, and silicon electronic components as are known and taught in the LED art. The cover may include a cavity to avoid contacting the cover to the thin-film layers of materials when the cover is affixed to the substrate. Alternatively, it is known to provide a polymer layer between the thin-film layers of materials and the cover.
While quantum dots may be useful and stable light emitters, in prior-art designs the emitted light may be trapped within the light-emitting structure employed to provide current or photo-stimulation to the quantum dots. Due to the high optical indices of the materials used, many of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the devices and make no contribution to the light output from these devices. Because light is emitted in all directions from the light-emitting layer, some of the light is emitted directly from the device, and some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
In the prior-art example of FIG. 2, a typical LED 11 structure is shown to contain an electroluminescent (EL) unit 16 between a first electrode 14 and second electrode 18. The EL unit 16 as illustrated contains all layers between the first electrode 14 and the second electrode 18, but not the electrodes themselves. Light-emitting layer 33 contains light-emitting quantum dots 39 in a semiconductor matrix 31. Semiconductor matrix 31 can be an organic host material in the case of hybrid devices, or a polycrystalline inorganic semiconductor matrix in the case of inorganic quantum dot LEDs. EL unit 16 can optionally contain p-type or n-type charge transport layers 35 and 37, respectively, in order to improve charge injection. EL unit 16 can have additional charge transport layers, or contact layers (not shown). One typical LED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), an EL unit 16 containing a stack of layers, and a reflective cathode layer. The layers in the EL unit can be organic, inorganic, or a combination thereof. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as a top-emitting device. In typical hybrid LED devices, the refractive indices of the ITO layer, the organic semiconductor layers, and the glass are about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions. For all inorganic devices, the situation is worse due to the higher refractive index of the EL unit—typically greater than or equal to 2.0.
Referring to FIG. 3a, an LED device as taught in the prior art includes a substrate 10 on which are formed thin-film electronic components 20, for example, conductors, thin-film transistors, and capacitors in an active-matrix device or conductors in a passive-matrix device. The thin-film electronic components 20 can cover a portion of the substrate 10 or the entire substrate 10, depending on the device design. Over the substrate 10 are formed one or more first electrode(s) 14. An EL unit 16 containing one or more layers of semiconductor materials is formed over the first electrode(s) 14, at least one layer of which is light emitting. One or more second electrode(s) 18 are formed over the EL unit 16. A cover 12 with a cavity forming a gap 32 to avoid contacting the thin-film layers 14, 16, and 18 is affixed to the substrate 10. In some designs, it is proposed to fill the gap 32 with a curable polymer or resin material to provide additional rigidity. The second electrode(s) 18 may be continuous over the surface of the electroluminescent device. Upon the application of a voltage across the first and second electrodes 14 and 18, respectively provided by the thin-film electronic components 20, a current can flow through the semiconductor material layers in EL unit 16 to cause one of the semiconductor layers to emit light 50a through the cover 12 (if the cover 12 and any material in the gap 32, and the second electrode 18 are transparent) or to emit light 50b through the substrate 10 (if the substrate 10 and the first electrode 14 are transparent). The light-emitting semiconductor layer contains light-emitting quantum dots. If light is emitted through the substrate 10, it is a bottom-emitter OLED; and the thin-film electronic components 20 may occlude some of the light 50b emitted or may limit the emission area to the area 26 between the thin-film electronic components 20, thereby reducing the aperture ratio of the LED device. If light is emitted through the cover 12, the LED device is a top-emitter; and the thin-film electronic components 20 do not necessarily occlude the emitted light. The arrangement used in FIG. 2 is typically a bottom emitter configuration with a thick, highly conductive, reflective electrode 18 and suffers from a reduced aperture ratio. Referring to FIG. 3b, a top-emitter configuration can locate a first electrode 14 partially over the thin-film electronic components 20 thereby increasing the amount of light-emitting area 26. Since, in this top-emitter case, the first electrode 14 does not transmit light, it can be thick, opaque, and highly conductive. However, the second electrode must then be transparent.
For example, in the commercial practice of similar OLED devices, the substrate and cover have comprised 0.7 mm thick glass, for example, as employed in the Eastman Kodak Company LS633 digital camera. For relatively small devices, for example, less than five inches in diagonal, the use of a cavity in a cover 12 is an effective way of providing relatively rigid protection to the thin-film layers of materials 16. However, for very large devices, the substrate 10 or cover 12, even when composed of rigid materials like glass and employing materials in the gap 32, can bend slightly and cause the inside of the cover 12 or gap materials to contact or press upon the thin-film layers of materials 16, possibly damaging them and reducing the utility of the LED device.
It is known to employ spacer elements to separate thin sheets of materials. For example, U.S. Pat. No. 6,259,204, by Ebisawa et al., granted Jul. 10, 2001, describes the use of spacers to control the height of a sealing sheet above a substrate. Such an application does not, however, provide protection to thin-film layers of materials in a thin-film LED device. US 2004/0027327 entitled, “Components and methods for use in electro-optic displays” published Feb. 12, 2004, describes the use of spacer beads introduced between a backplane and a front plane laminate to prevent extrusion of a sealing material when laminating the backplane to the front plane of a flexible display. However, in this design, any thin-film layers of materials are not protected when the cover is stressed. Moreover, the sealing material will reduce the transparency of the device and requires additional manufacturing steps.
U.S. Pat. No. 6,821,828, by Ichijo et al, granted Nov. 23, 2004, describes an organic resin film, such as an acrylic resin film patterned to form columnar spacers in desired positions in order to keep two substrates apart. The gap between the substrates is filled with liquid crystal materials. Spherical spacers sprayed onto the entire surface of the substrate may replace the columnar spacers. Similarly, U.S. Pat. No. 6,559,594, granted May 6, 2003, by Fukunaga et al., describes the use of a resin spacer formed on the inside of the cover of an EL device. However, such a resin spacer may de-gas and requires expensive photolithographic processing and may interfere with the employment of color filters. Moreover, columnar spacers are formed lithographically and require complex processing steps and expensive materials. Moreover, this design is applied to liquid crystal devices and does not provide protection to thin-film structures deposited on a substrate. Additionally, rigid non-compressible spacers will transfer applied pressure directly to underlying layers, potentially damaging them.
U.S. Pat. No. 6,551,440 entitled “Method of manufacturing color electroluminescent display apparatus and method of bonding light-transmitting substrates,” granted Apr. 22, 2003. In this invention, a spacer of a predetermined grain diameter is interposed between substrates to maintain a predetermined distance between the substrates. When a sealing resin, deposited between the substrates, spreads, surface tension draws the substrates together. The substrates are prevented from being in absolute contact by interposing the spacer between the substrates, so that the resin can be smoothly spread between the substrates. This design does not provide protection to thin-film structures deposited on a substrate.
The use of cured resins is also optically problematic for top-emitting LED devices. As is well known, a significant portion of the light emitted by an LED may be trapped in the LED layers, substrate, or cover. By filling the gap with a resin or polymer material, this problem may be exacerbated. Referring to FIG. 6, a prior-art bottom-emitting LED has a transparent substrate 10, a transparent first electrode 14, EL unit 16, a reflective second electrode 18, a gap 32 and an encapsulating cover 12. The encapsulating cover 12 may be opaque and may be coated directly over the second electrode 18 so that no gap 32 exists. When a gap 32 does exist, it may be filled with polymer or desiccants to add rigidity and reduce water vapor permeation into the device. Light emitted from EL unit 16 can be emitted directly out of the device, through the substrate 10, as illustrated with light ray 1. Light may also be emitted and internally guided in the substrate 10 and EL unit 16, as illustrated with light ray 2. Alternatively, light may be emitted and internally guided in EL unit 16, as illustrated with light ray 3. Light rays 4 emitted toward the reflective second electrode 18 are reflected by the reflective second electrode 18 toward the substrate 10 and then follow one of the light ray paths 1, 2, or 3.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the emissive layers; see “Modification of polymer light emission by lateral microstructure” by Safonov et al., Synthetic Metals 116, 2001, pp. 145-148, and “Bragg scattering from periodically microstructured light emitting diodes” by Lupton et al., Applied Physics Letters, Vol. 77, No. 21, Nov. 20, 2000, pp. 3340-3342. Brightness enhancement films having diffractive properties and surface and volume diffusers are described in WO/0237568, entitled “Brightness and Contrast Enhancement of Direct View Emissive Displays” by Chou et al., published May 10, 2002. The use of micro-cavity techniques is also known; for example, see “Sharply directed emission in organic electroluminescent diodes with an optical-microcavity structure” by Tsutsui et al., Applied Physics Letters 65, No. 15, Oct. 10, 1994, pp. 1868-1870. However, none of these approaches cause all, or nearly all, of the light produced to be emitted from the device. Moreover, such diffractive techniques cause a significant frequency dependence on the angle of emission so that the color of the light emitted from the device changes with the viewer's perspective.
Reflective structures surrounding a light-emitting area or pixel are referenced in U.S. Pat. No. 5,834,893, issued Nov. 10, 1998 to Bulovic et al. and describe the use of angled or slanted reflective walls at the edge of each pixel. Similarly, Forrest et al. describe pixels with slanted walls in U.S. Pat. No. 6,091,195 issued Jul. 18, 2000. These approaches use reflectors located at the edges of the light emitting areas. However, considerable light is still lost through absorption of the light as it travels laterally through the layers parallel to the substrate within a single pixel or light emitting area.
Scattering techniques are also known. Chou (WO 02/37580) and Liu et al. (U.S. Patent Application Publication No. 2001/0026124 A1) taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has an optical index that matches these layers. Light emitted from the LED device at higher than the critical angle that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device. The efficiency of the LED device is thereby improved but still has deficiencies as explained below.
U.S. Pat. No. 6,787,796 entitled, “Organic electroluminescent display device and method of manufacturing the same” by Do et al., issued Sep. 7, 2002, describes an organic electroluminescent (EL) display device and a method of manufacturing the same. The organic EL device includes a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, wherein a light loss preventing layer having different refractive index areas is formed between layers of the organic EL device having a large difference in refractive index among the respective layers. U.S. Patent Application Publication No. 2004/0217702 entitled “Light extracting designs for organic light emitting diodes” by Garner et al., similarly discloses use of microstructures to provide internal refractive index variations or internal or surface physical variations that function to perturb the propagation of internal waveguide modes within an OLED. When employed in a top-emitter embodiment, the use of an index-matched polymer adjacent to the encapsulating cover is disclosed.
However, scattering techniques, by themselves, cause light to pass through the light-absorbing material layers multiple times where they are absorbed and converted to heat. Moreover, trapped light may propagate a considerable distance horizontally through the cover, substrate, or organic layers before being scattered out of the device, thereby reducing the sharpness of the device in pixellated applications such as displays. For example, as illustrated in FIG. 7, a prior-art pixellated bottom-emitting LED device may include a plurality of independently controlled pixels 60, 62, 64, 66, and 68 and a scattering element 21, typically formed in a layer, located between the transparent first electrode 14 and the substrate 10. A light ray 5 emitted from the light-emitting layer may be scattered multiple times by light scattering element 21, while traveling through the substrate 10, EL unit 16, and transparent first electrode 14 before it is emitted from the device. When the light ray 5 is finally emitted from the device, the light ray 5 has traveled a considerable distance through the various device layers; from the original pixel 60 location, where it originated, to a remote pixel 68 where it is emitted, thus reducing sharpness. Most of the lateral travel occurs in the substrate 10, because that is by far the thickest layer in the package. Also, the amount of light emitted is reduced due to absorption of light in the various layers.
Light-scattering layers used externally to an OLED device are described in U.S. Patent Application Publication No. 2005/0018431 entitled, “Organic electroluminescent devices having improved light extraction” by Shiang and U.S. Pat. No. 5,955,837 entitled, “System with an active layer of a medium having light-scattering properties for flat-panel display devices” by Horikx, et al. These disclosures describe and define properties of scattering layers located on a substrate in detail. Likewise, U.S. Pat. No. 6,777,871 entitled “Organic ElectroLuminescent Devices with Enhanced Light Extraction” by Duggal et al., describes the use of an output coupler comprising a composite layer having specific refractive indices and scattering properties. While useful for extracting light, this approach will only extract light that propagates in the substrate (illustrated with light ray 2) and will not extract light that propagates through the organic layers and electrodes (illustrated with light ray 3). Moreover, if applied to display devices, this structure will decrease the perceived sharpness of the display. A device with the light-scattering layer is much less sharp than the device without the light scattering layer, although more light is extracted from an LED device with the light-scattering layer.
U.S. Patent Application Publication No. 2004/0061136 entitled, “Organic light emitting device having enhanced light extraction efficiency” by Tyan et al., describes an enhanced light extraction OLED device that includes a light scattering layer. In certain embodiments, a low-index isolation layer (having an optical index substantially lower than that of the organic electroluminescent element) is employed adjacent to a reflective layer in combination with the light scattering layer to prevent low angle light from striking the reflective layer, and thereby reduce absorption losses due to multiple reflections from the reflective layer. The particular arrangements, however, may still result in reduced sharpness of the device.
There is a need therefore for an improved LED device structure that improves the robustness of the device and reduces manufacturing costs; this solution preferably simultaneously improves device performance.